How it works...

The Ambient Occlusion option will calculate areas that should have additional shadows. Since our scene is filled with trees, this will make the undersides much darker than they were previously.

For more information on the Ambient Occlusion effect, check out: https://github.com/Unity-Technologies/PostProcessing/wiki/Ambient-Occlusion. If you are interested in looking into the other options that the Post Processing stack has, check out: https://github.com/Unity-Technologies/PostProcessing/wiki.